This invention relates to cast components which define the combustion chambers of internal combustion engines, such as pistons and cylinder heads, which are preferably made of light alloys and in which those surface portions which are subjected to wear and/or to high temperatures contain an embedded reinforcing layer consisting of nonwoven ceramic fibers or of whiskers. The invention also relates to a process of joining the reinforcing elements to the components.
Owing to the considerably increase of the power per unit of weight and of the life, reliability in operation and economy of internal combustion engines, their components, particularly the components which define the combustion chamber, such as the pistons and cylinder head, must be so designed that they will withstand the stresses to be encountered. The stress of the piston is mainly caused by the mechanical loading due to the gas pressure and the thermal loading which is due to high temperatures and to stresses imposed by the temperature gradient. For a control of the high thermal loading the piston may be provided with a cooling chamber but the latter usually does not tend to reduce the thermal stresses because the temperature gradient is increased adjacent to the cooling chamber. A cooling of the piston is also inconsistent with the desire to reduce the dissipation of heat from the combustion process of the coolant and to the lubricant although that desire involves a high thermal loading of the piston head.
A piston having a piston head which defines a combustion recess will be particularly critically stressed at the rim of said recess. In that zone the heat transfer is decreased by the accelerated flow of the combustion gases but the dissipation of heat is restricted. These influences tend to increase the temperature at the rim of the recess and together with the high tangential stress which is due to the temperature distribution result in a stress above the elastic limit. A cooling which results from a load chamber or from a shutdown of the internal combustion engine will result in inherent tensile stresses. As that phenomenon occurs frequently, thermal fatigue cracks are formed at the rim of the recess and may grow to a large depth. In pistons having a cooling chamber said cracks may finally open into the cooling chamber so that the combustion gases can be blown into said chamber.
The internal combustion engine may have such a design that abnormally high temperatures are obtained in the ring zone or a fuel may be used which leaves large amounts of residues, or a large piston clearance may have been selected for special reasons so that the piston can tilt, or the air may not be adequately filtered, particularly in a dust-laden environment, e.g., on building sites. Said conditions will render the ring grooves of the piston, particularly the uppermost ring groove, susceptible to wear, which may result in a risk of a breakage of the piston rings.
A restricted relief of the load on the piston head and the rim of the recess may be achieved by an optimum design and by a decrease of the stresses which are due to the gas force and the temperature distribution.
That problem can be remedied for a short operating time in that the rim of the recess is reinforced by an embedded ring consisting of nickel-alloyed cast iron (Fachkunde Kraftfahrtechnik, 2nd edition, Holland + Josenhans Verlag, Stuttgart 1982, page 27). But after a prolonged operation the permanent deformation results in a formation of a gap between the cast iron ring and the piston head. Other disadvantages of such reinforcement at the rim of the recess reside in that it adds to the weight and to the manufacturing costs of the piston. For this reason it has been proposed to provide a ring made of aluminum titanate (Published German Application No. 32 30 388). But owing to the shrinkage joint between the ring and the surrounding piston material such ring can be used only if it can bear on a sufficiently large surface of the surrounding piston material. In the search for alternative solutions which avoid the disadvantages mentioned above it has been proposed to apply metallic and cement layers to the piston head by flame spraying and plasma spraying (Published German Application No. 31 37 731). But the deposition of such layers is disturbed at the sharp edges of the rim of the combustion recess and the layers do not adequately adhere under special loads. This is due to the high thermal loading which are encountered and which gradually soften the material and promote the formation of cracks, and to the high temperature gradient which is obtained in the coating and results in high stresses (Published German Application No. 25 07 899). From Published German Application No. 25 07 899 it is known to provide piston heads with a hard layer of anodized aluminum in a thickness of 30 to 80 microns. Whereas this results in a distinctly higher resistance to thermal fatigue, an appreciable temperature drop is not effected thereby. Another known piston (Diesel Progress North America, April 1984) is provided with a sheet steel covering to which a steel wire mesh has been sintered on the side which is remote from the stressed side. The covering is joined to the piston as it is made by pressure diecasting. That piston effects only a limited heat insulation and its weight is increased by the covering and by the steel wire mesh.
If there is a risk of wear of the ring grooves, so-called ring carrier pistons have been used in which a cast piston body consisting of an aluminum piston alloy is provided at least adjacent to the uppermost piston ring groove with an embedded ring carrier, which is made of ferrous material and is formed with a recess consisting of the ring groove and said ring carrier is joined to the piston body by means of an intermetallic bonding layer (Thecnisches Handbuch, Karl Schmidt GmbH, Neckarsulm 1967, pages 106 to 107). The ring carrier embedded in the cast piston body adds undesirably to the weight of the aluminum piston. Another reinforcement is provided in that a piston consisting of an aluminum alloy is formed with a groove in the ring zone and said groove is lined by metal spraying with a thin interlayer of molybdenum, cobalt, nickel or an alloy thereof and of stainless steel and is then filled with a stainless steel having a high chromium content and a specified nickel content, and the ring grooves are subsequently formed (Published German Application No. 14 00 115). The disadvantage of that practice resides in that the bond strength between the aluminum layer and the sprayed interlayer is low.
British Patent Specification No. 2,106,433 describes a light alloy piston which is made by pressure diecasting and in which a layer of fibers or whiskers is embedded in the piston head in order to avoid a formation of cracks. In Published European Patent Application No. 80 562 it is proposed to effect a protection against wear and/or a formation of thermal fatigue cracks in light alloy components in that non-woven ceramic fibers in a proportion of 2 to 10 vol. % are embedded in such components. But such pistons or components are not satisfactory as regards wear resistance and heat insulation.